1. Technical Field
The present invention relates generally to a steering control system for a vehicle. More particularly, the invention relates to a rear wheel steering control system for determining rear wheel steering angle according to a degree of driven wheel slippage to improve steering stability when turning.
2. Background Art
It is well known in the art that four-wheel steering (4WS) vehicles are adapted for providing cornering forces on rear wheels during turns without slipping to improve turning stability.
A Japanese Patent First Publication No. 57-60974 discloses a rear wheel steering system. This system is operable to detect acceleration or centripetal force acting on a vehicle body during turns to steer rear wheels to the same phase as a steered angle of front wheels in the case of a front-wheel drive (FWD) vehicle or to a phase opposite the front wheels in the case of a rear-wheel drive (RWD) vehicle for preventing spin of the FWD vehicle from occurring which is caused by driven wheel slippage during turning with acceleration or, in the case of rear-wheel drive vehicles preventing drift-out from occurring.
The above prior art steering system is however provided with an acceleration sensor which is installed on a part of the vehicle body to detect acceleration including an unnecessary component induced by vehicle motion such as vertical displacement or pitching due to uneven road or acceleration, resulting in unsuitable rear wheel steering control.
For avoiding such drawbacks, a Japanese Patent First Publication No. 62-71761 discloses a rear wheel steering system which controls a steering operation for rear wheels based on a difference in rotational speeds between driven and compliance wheels to prevent the occurrence of spin or drift-out due to driven wheel slippage (i.e., due to rotational speed of the driven wheels being higher than that of the compliance wheels), during turning with acceleration.
The driven wheel slip is detected by determining the difference in rotational speeds between the driven and compliance wheels. As there is a difference in rotational speeds between front and rear wheels induced by a difference in turning radius therebetween in addition to the rotational speed difference due to the driven wheel slip, the rotational speed difference due to the turning radius difference impairs the rear wheel steering control greatly during turns with a minimum turning radius wherein the turning radius difference becomes great.